1. Field of the Invention
The invention relates to the field of scanning probe microscopy cantilevers and read-out schemes for detecting and monitoring movements of the cantilevers when the latter is operated above the surface of a material.
2. Description of Related Art
Scanning probe microscopy (SPM) implements scanning tunneling and the atomic force microscope. It aims to form images of sample surfaces using a physical probe. Scanning probe microscopy techniques rely on scanning such a probe, e.g. a sharp tip, just above or in contact with a sample surface whilst monitoring interaction between the probe and the surface. An image of the sample surface can thereby be obtained. Typically, a raster scan of the sample is carried out and the probe-surface interaction is recorded as a function of position. Data are typically obtained as a two-dimensional grid of data points.
The resolution achieved varies with the underlying technique. Atomic resolution can be achieved in some cases. Typically, either piezoelectric actuators or electrostatic actuation are used to execute precise motions of the probe.
Two main types of SPM are the scanning tunneling microscopy (STM) and the atomic force microscopy (AFM). The invention of STM was quickly followed by the development of a family of related techniques (including AFM), which together with STM form the SPM techniques. As known, the interaction monitored in STM is the current tunneling between a metallic tip and a conducting substrate. The quantum mechanical concept of tunneling allows for electrons to tunnel through a potential barrier, which they cannot surmount according to the paradigm of classical physics. Thus, in the quantum world, electrons are able to hop through the classically-forbidden space between the tip and the sample.
Using STM, imaging of the surface topology is usually carried out in one of two modes: (i) in constant height mode, wherein the tunnel current is monitored as the tip is moved parallel to the surface); and (ii) in constant current mode, wherein the tunnel current is maintained constant as the tip is scanned across the surface and a deflection of the tip is measured.
In AFM techniques, forces between the tip and the surface are monitored. This can be either the short range Pauli repulsive force (in contact-mode) or the longer range attractive force (in non-contact mode, merely van der Waals forces).
Using AFM techniques, imaging of the surface topology is usually carried out in one of three modes: (i) in contact mode, where the probe is moved over the surface with constant contact thus monitoring the surface by changing the height set-point; and (ii) in non-contact mode, where a stiff cantilever oscillates with a small amplitude of typically less than 10 nm above the surface. Influences of the surface lead to changes in frequency and amplitude of the cantilever. These changes can be detected and used as the feedback signals. In a third mode, (iii) the intermittent contact or tapping mode, the cantilever is oscillated with larger amplitude of typically 100 to 200 nm. Short range forces are detectable without sticking of the cantilever to the surface.
The above techniques are translated into topography by a sensor. A common type of sensor is a bulk-component-based free-space laser beam deflection setup with a four quadrant photo diode acting as the deflection sensor. Other known principles include thermal height sensing and piezoresistive deflection sensors.
In both STM and AFM, the position of the tip with respect to the surface must be accurately controlled (e.g., to within about 0.1 Å) by moving either the sample or the tip. The tip is usually very sharp; ideally terminated by a single atom or molecule at its closest point to the surface.
Metallic probe tips for conductive measurements are typically made of platinum/iridium or gold. In this respect, two main methods for obtaining a sharp probe tip are known: acid etching and cutting. The first method involves dipping a wire end first into an acid bath and waiting until it has etched through the wire and the lower part drops away. The resulting tip can thus often be one atom in diameter at its end. An alternative and quicker method is to take a thin wire and cut it with convenient tools. Testing the tip produced via this method on a sample with a known profile will then indicate whether the tip is suitable or not.
Silicon probe tips as typically used for non-conductive AFM measurements are typically made by isotropically etching a silicon pillar structure until the required sharpness is reached.
For the sake of exemplification, U.S. Pat. No. 5,059,793 (A) provides a scanning type tunnel microscope in which a servo system for controlling the distance between the probe and the sample can be set in a proper condition irrespective of the surface condition of the sample. As another example, U.S. Pat. No. 5,546,375 (A) provides a method for manufacturing a fine tip for detecting a fine current or force.
A number of publications are directed to STM and the manufacture of SPM probes, see e.g., Hayashi, T., Tachiki, M., Itozaki, H., Applied Superconductivity, IEEE Transactions on Volume 17, Issue 2, June 2007 Page(s): 792-795 (DOI 10.1109/TASC.2007.898557).
In other technical fields, one also knows optical waveguides, ring resonators, isolators, and other optical components which are basic building blocks for a number of integrated photonic components, such as switches, lasers, filters and sensors. Optical ring resonators comprise a waveguide in a closed loop, typically coupled to input/output waveguides. Light having an appropriate wavelength can be coupled to the loop via the input waveguide, to build up in intensity over multiple round-trips, owing to constructive interference. Light can then be picked up via an output waveguide. Also, since a determined set of wavelengths resonate in the loop, the resonator can be used as a filter.